1. Field of the Invention
This invention relates to particular copolymers of acrylic acid and itaconic acid which are at least water dispersible and their use as fluid loss control additives for aqueous drilling fluids which are used in drilling wells in subterranean formations.
2. Description of the Prior Art
Rotary systems are generally used in drilling oil and gas wells. These systems depend upon rotation of a string of drill pipe to the bottom of which is attached a drill bit. The bit cuts into the formation causing the cuttings to accumulate as drilling continues. A drilling fluid is used which lubricates the drill bit and carries cuttings to the surface for removal. This allows the bit to continue functioning and the bottom hole to be clean and free of cuttings. The drilling fluid is also the source of pressure which is exerted against the formation. Even in other drilling systems, drilling fluids are still needed to remove bore hole cuttings and to otherwise perform functions related to drilling fluids.
Oil and gas producing formations are generally porous layers having varying degrees of permeability to the flow of fluids such as oil, water or gas. When drilling through such porous formations, it is essential to employ drilling fluids having characteristics such that excessive amounts of the drilling fluids do not penetrate the porous formation. Drilling fluids have a tendency to penetrate the formation because pressure in the bore hole is greater than the pressure opposing it which comes from the formation. Should excess penetration occur, there is loss of drilling fluid into the formation resulting in loss of pressure, inability to drill and remove cuttings and in the case of loss from brine muds, extra expense because of their cost. Mere filter cake formation on the wall of the bore hole does not adequately eliminate fluid loss.
According to U.S. Pat. No. 4,293,427, Lucas et al, Oct. 6, 1981, various materials have been added to drilling fluids to control fluid loss and have included materials such as pregelatinized starch, sodium carboxymethyl cellulose, sodium polyacrylates and lignites.
U.S. Pat. No. 2,718,497--Oldham et al, Sept. 20, 1955, discloses that polymers of acrylic acid when added to drilling muds reduce fluid loss of same. The muds to which the polymer is added do not contain calcium or chloride ions.
C. S. Stanley in World Oil, July 1959 (pages 122-128) describes studies, including fluid loss studies, of acrylic acid-acrylamide copolymers of various carboxylate/amide ratios and molecular weights on mud formulations.
C. S. Stanley in American Chemical Society, Division Petroleum Chemical Preprints 7(1), pages 65-70 (1962), describes fluid loss control studies on clay-water suspensions used as oil well drilling fluids using a copolymer of about 200,000 molecular weight consisting of about 70 mole % sodium acrylate and 30 mole % acrylamide.
CA 87:25579e (1977) discloses a water loss reducer and stabilizer of drilling fluids prepared by partial hydrolysis of an acrylonitrile itaconic acid copolymer.
CA 52:20981h (1958) discloses salts of polymethacrylic or polyacrylic acid used as modifiers for cement or mortar, including salts of copolymers of methacrylic or acrylic acid and maleic, fumaric or itaconic acid.
CA 93:120386x (1980) discloses synthesis of polycarboxylic acids and the properties of cement formed by their reaction with ion-leachable glasses. A copolymer of acrylic and itaconic acids combined several favorable characteristics.
CA 99:110801p (1938) discloses dental cements containing glass particles coated with polymers. Acrylic acid-itaconic acid copolymer is disclosed.
Where polyacrylates are added to fresh water muds, fluid loss can be reasonably controlled. However, where soluble calcium or chloride ions are present in the muds, generally in excess of about 300 ppm of soluble calcium ions and in excess of about 1,000 ppm of chloride ions, as for example from the cement or from the formation in the case of the former and from seawater (brine) and salt water in the case of the latter, particularly under conditions of high temperature and pressure, polyacrylates are not as effective in controlling fluid loss.